Known as carboxyl group-containing polymers to be used as thickening agents for cosmetics and the like, as humectants for cataplasms, and as suspension stabilizers for emulsions, suspensions and the like, among others, are crosslinking type carboxyl group-containing polymers and alkyl-modified carboxyl group-containing polymers, among others. Known as the crosslinking type carboxyl group-containing polymers are, for example, copolymers of an α,β-unsaturated carboxylic acid, such as acrylic acid, and a polyallyl ether (cf. Patent Document 1), copolymers of an α,β-unsaturated carboxylic acid and hexaallyl trimethylene trisulfone (cf. Patent Document 2), copolymers of an α,β-unsaturated carboxylic acid and glycidyl methacrylate or the like (cf. Patent Document 3), copolymers of an α,β-unsaturated carboxylic acid and pentaerythritol allyl ether (cf. Patent Document 4, Patent Document 5 and Patent Document 6), and copolymers of an α,β-unsaturated carboxylic acid, a (meth)acrylic acid ester and pentaerythritol allyl ether (cf. Patent Document 7 and Patent Document 8), among others.
Known as the alkyl-modified carboxyl group-containing polymers are, for example, copolymers of polyacrylic acid or (meth)acrylic acid and a (meth)acrylic acid alkyl ester, among others. More specifically, there are known copolymers obtained by reacting a specific amount of an olefinically unsaturated carboxylic acid monomer with a specific amount of a (meth)acrylic acid alkyl ester (the alkyl group containing 10 to 30 carbon atoms) (cf. Patent Document 9) and copolymers obtained by reacting an olefinically unsaturated carboxylic acid monomer with a (meth)acrylic acid alkyl ester (the alkyl group containing 8 to 30 carbon atoms) (cf. Patent Document 10).
In using the carboxyl group-containing polymer particle comprising a crosslinking type carboxyl group-containing polymer, an alkyl-modified carboxyl group-containing polymer or the like in the above-mentioned fields of application, it is first necessary to prepare a uniform aqueous dispersion of the carboxyl group-containing polymer particles and then neutralize the dispersion with an alkali to give a neutralized viscous liquid with a concentration of about 0.1 to 1% by weight. However, the above-mentioned carboxyl group-containing polymer particles are generally in the form of a fine powder and, therefore, undissolved lumps are readily formed on the occasion of dispersing the particles in water. There is a drawback that once undissolved lumps have been formed, a gel-like layer is formed on the surface of each lump and therefore the rate of permeation of water into the inside thereof is reduced and it becomes difficult to obtain a uniform aqueous dispersion.
Therefore, in preparing an aqueous dispersion of carboxyl group-containing polymer particles, a procedure which comprises gradually adding the carboxyl group-containing polymer particles in powder form to water with high-speed stirring and therefore is poor in production efficiency is required and, in some instances, a special dissolution apparatus is required for preventing the formation of undissolved lumps.
Furthermore, the carboxyl group-containing polymer particles mentioned above are in the form of a fine powder and are readily charged electrically and therefore cause heavy dusting. Therefore, the carboxyl group-containing polymer particles mentioned above are not only difficult to handle but also are disadvantageous from the working environment viewpoint. A further problem is that the carboxyl group-containing polymer particles in fine powder form have low bulk density and therefore cause increases in transportation cost and require an increased number of storage sites.
Therefore, the advent of a granular carboxyl group-containing polymer particle has been awaited.
In granulating a powder, a granulator is generally used. The granulators are broadly classified into three types, that is, mixing granulation, forced granulation and granulation utilizing heat.
As granulators for mixing granulation, there may be mentioned a fluidized bed granulator, a tumbling granulator and the like, which produce granules by causing a powder to float or flow by means of an air stream or stirring blade or by tumbling while uniformly spraying the powder with a liquid to serve as a binder. When an attempt is made to apply these granulators for mixing granulation to the carboxyl group-containing polymer particle, it is difficult to cause the carboxyl group-containing polymer particles to float or flow by means of an air stream or stirring blade or by tumbling since it is very small in particle diameter and low in bulk density. As for the liquid to be used as the binder, water is preferred from the viewpoint that the carboxyl group-containing polymer particle is mainly used in cosmetic preparations and the viewpoint of the environmental friendliness. However, when the floating or flowing carboxyl group-containing polymer particles are sprayed with water, the carboxyl group-containing polymer particles acquire tackiness and adhere to the granulator inside wall and binder-spraying nozzle or sticks to one another to form large undissolved lumps.
As granulators for forced granulation, there may be mentioned a compression molding granulator, an extrusion granulator and the like. As an example of granulation of the carboxyl group-containing polymer particle utilizing forced granulation, the method is known which comprises compressing the fine powder using a compression molding machine, followed by grinding (cf. Patent Document 11). However, this method has problems that, for example, the carboxyl group-containing polymer particles are excessively compressed by the pressure exerted by the compression molding machine and the resulting granular carboxyl croup-containing polymer particles become poor in swelling property in water.
As a granulator for granulation utilizing heat, there may be mentioned a spray dryer. However, this has problems that, for example, it is economically-disadvantaged due to the need to dilute the carboxyl group-containing polymer particles with water, an organic solvent or the like to a viscosity level at which the diluted product can be sprayed and, in addition, the granule obtained is not porous and therefore poor in solubility in water.
On the other hand, methods are known for granulation without using such granulators as mentioned above, for example, the method comprising bringing a polymer flocculant in fine powder form into contact with water vapor to thereby effect granulation (cf. Patent Document 12), the method comprising dispersing a water-soluble polymer in fine powder form in an organic solvent and then adding water to thereby effect granulation (cf. Patent Document 13) and the method comprising feeding a lubricant and water simultaneously and continuously to a water-soluble polymeric substance in fine powder form and grinding the thus-granulated gel bodies (cf. Patent Document 14). In the case of the carboxyl group-containing polymer particle, however, it is difficult to apply these methods because of such problems as (1) the problem that the carboxyl group-containing polymer particles, when brought into contact with water formed due to dew condensation of water vapor, swell with that water and the finally-obtained granule is not porous and therefore poor in solubility in water, in addition, the problem that, in the case of the method comprising allowing a powder to fall to thereby bring the same into contact with water vapor, it is difficult to allow the carboxyl group-containing polymer particles, which are poor in flowability, to fall uniformly and a large amount of dust may possibly be flung up on the occasion of allowing the powder to fall, (2) the problem that, on the occasion of adding water following dispersing in an organic solvent, the resulting hydrous gel forms undissolved lumps and (3) the problem that the granule contains an unnecessary lubricant depending on use application and is produced via a gel form, resulting in that the granule obtained is not porous and therefore poor in solubility in water.
Patent Document 1: U.S. Pat. No. 2,923,629
Patent Document 2: U.S. Pat. No. 2,958,679
Patent Document 3: Japanese Kokai Publication Sho-58-84819
Patent Document 4: U.S. Pat. No. 5,342,911
Patent Document 5: U.S. Pat. No. 5,663,253
Patent Document 6: U.S. Pat. No. 4,996,274
Patent Document 7: Japanese Kokoku Publication Hei-5-39966
Patent Document 8: Japanese Kokoku Publication Sho-60-12361
Patent Document 9: Japanese Kokai Publication Sho-51-6190
Patent Document 10: U.S. Pat. No. 5,004,598
Patent Document 11: International Publication WO 03/016382
Patent Document 12: Japanese Kokai Publication Sho-52-2877
Patent Document 13: Japanese Kokai Publication Sho-52-136262
Patent Document 14: Japanese Kokai Publication Hei-3-143605